1. Field of the Invention
The present invention relates to an image reading apparatus which measures an image reading position error from bit-map formation image data obtained from reading an image.
2. Description of the Related Art
As a first example of the related art, Shunsuke Hattori et al. disclose A Development of Image Scanner of High Resolution in the Japan Society of Mechanical Engineers, 71st ordinary general meeting, lecture meeting, lecture paper collection (IV) Mar. 29-31, 1994, Tokyo!. Therein, an interpolation operation is performed on image data which is obtained as a result of reading a test chart having even-pitch lines arranged therein in a sub-scan direction. That image data is image data which is discrete in those sub-scan direction line intervals. From the interpolation operation result, the central positions of black lines and white lines of the even-pitch lines are obtained. Then, differences between the central positions and the reference pitch of the test chart are read. Thereby, image data reading position errors due to apparatus vibration or the like is detected.
As a second example of the related art, Japanese Laid-Open Patent Application No. 6-297758 discloses A Scan-line Pitch Measuring Method. Therein, a pattern of a hard copy having even-pitch pattern data written therein is read. Thereby, unevenness in the pitch of scan lines which are used in a hard-copy apparatus is measured.
In the above-described first example of the related art, due to possible spatial differences between the edges of the even-pitch lines and sampling positions, a `Moire` effect may occur wherein a difference occurs between data which has been obtained as a result of reading the same pattern. Due to the Moire effect, thus-obtained read data may not be data which indicates positions corresponding to the edges of the pattern. Thereby, accuracy in measuring image reading position errors may be degraded. Such an adverse effect is very noticeable when the even-pitch line pattern is so fine as to approximate the resolution of the reading apparatus. As a result, the measuring of image reading position errors may not be performed. Thus, using this method, it is not possible to measure, with a high accuracy, an image reading position error of a pattern which is so fine as to approximate or to exceed than the resolution of the reading apparatus.
Further, because an even-pitch line pattern is used, even if the Moire effect is ignored, in a case where the pitch of the pattern is fine for measuring an image reading position error of a high-frequency component, due to the limitation of the MTF (Modulation Transfer Ratio) of the image formation system, a difference in a signal indicating image tone is disadvantageously reduced. Thus the measuring accuracy is degraded.
It is considered that, in the case where the pitch of the pattern is finer, the measuring frequency band is widened to a higher frequency. Thereby, it is not possible to provide a high measuring accuracy. Therefore, in order to solve this problem, the sampled data is made to undergo an interpolation operation. In order to improve the effect of the interpolation operation, it is necessary to increase an amount of surrounding data to be processed and to perform a complicated calculation. As a result, a longer time is required for the calculation. Further, the interpolation operation inherently may not provide true data, and thus the measuring accuracy may be degraded. Further, in the first example, image data to be used is obtained as a result of a specific light-reception element of the light-to-electricity converting device being used to scan the pattern in the sub-scan direction. The light-reception element itself may provide noise which may degrade the measuring accuracy.
In the above-described method of measuring in the second example of the related art, the light-to-electricity converting device is used to read the pattern and thus-obtained data is used. In this method, at this time, reading or scanning unevenness when reading or scanning a hard copy is not considered in measuring pitch unevenness in the pattern of the hard copy. Further, this method also has a `Moire` effect problem similar to that which occurs in the above-described first example of the related art.
In order to solve these problems, the applicant of the present application filed Japanese Patent Application No. 7-260438. In this application, a pattern which is formed of a plurality of lines which have a fixed inclination with respect to a scanning direction is set on an image reading apparatus. A position deviation of the pattern may occur when the pattern is set on the image reading apparatus. A technique of detecting such a position deviation of the pattern was proposed in the above-mentioned application. In this technique, in order to correct such position deviation which occurs when the oblique-line pattern is set, an inclination of the oblique-line pattern is measured by linear approximation from measured image reading position error data.
However, when the average of a scanner's scanning speed is not a predetermined value, image reading position error data also has an inclination. Accordingly, it is necessary to separate the inclination of image reading position error data due to a position deviation of the oblique-line pattern and an inclination of the image reading position error data due to deviation of the average of a scanner's scanning speed from one another.